Field of the Invention
The invention lies in the automotive technology field. More specifically, the invention relates to a method of correcting the characteristic of a linear lambda probe arranged upstream of a catalytic converter in an emission control system of an internal combustion engine.
In internal combustion engines which operate according to the spark-ignition principle (Otto engines), the quality of the combustion air is usually set via a throttle valve arranged in the intake pipe. However, it is also possible to control the inlet valves such that these, for example, electromechanical valves take over this function.
In order to clean the exhaust gas of an internal combustion engine, a three-way catalytic converter is usually arranged in the exhaust gas tract of the internal combustion engine. Provided upstream of this catalytic converter is a linear lambda probe whose output signal is dependent on the residual oxygen component contained in the exhaust gas. The residual oxygen component, in turn, is dependent on the mixture which has been fed to the internal combustion engine. The residual oxygen component in the raw exhaust gas is lower for fuel excess (rich mixture), and higher for air excess (lean mixture).
A linear lambda probe supplies a unique, monotonically rising signal in a wide lambda range (0.7 to 4). The signal is converted into a lambda value by means of a characteristic curve in a control unit. The internal combustion engine is controlled such that the lambda probe displays a value assigned to lambda=1. Since a three-way catalytic converter shows optimum catalytic properties for a raw exhaust gas with lambda=1, the predetermined mean value or the value assigned to lambda=1 should then also actually correspond to lambda=1. In other words, the characteristic must contain the correct assignment of signal and lambda value.
The dynamic and static properties of the lambda probe upstream of the three-way catalytic converter undergo change through aging and poisoning, however. As a result, the probe signal corresponding to lambda=1 shifts its position. In order to be able to correct this, it is known in accordance with the prior art to arrange downstream of the three-way catalytic converter a further lambda probe which is used as monitor probe to monitor the catalytic conversion and permits fine control of the mixture. For this purpose, the assignment of the signal of the lambda probe to the lambda value is corrected so that the lambda value most favorable for the conversion can always be observed. That method is referred to as guiding control or trimming control.
The correction therefore corresponds to shifting the characteristic curve. However, because of aging or a certain component tolerance, the gradient of the characteristic of the linear lambda probe can also deviate from the characteristic stored in the control unit. The consequence of such a deviation is that in the case of lambda values not equal to 1 the control unit converts the signal of the lambda probe into a defective lambda value. This error caused by an erroneous characteristic gradient becomes larger the more the lambda value of the exhaust gas deviates from lambda=1. This error can assume intolerable magnitudes, particularly in the case of lean-burn operation of the internal combustion engine.